The conviction of Amanda Knox has been thrown into question after a juror said she had 'grave doubts' over her involvement in the murder of British exchange student Meredith Kercher.

Genny Ballerini, one of the citizen panel judges at the last trial, has slammed the prosecution's case citing 'questionable proof, flimsy evidence and bizarre testimony'.

Her comments come just a week before Italy’s highest court will hear the appeal of Knox and her former boyfriend Raffaele Sollecito.

Knox, along with her former boyfriend Raffaele Sollecito, was convicted for a second time of the Leeds University student’s death in Perugia, Italy, in January last year.

Tragedy: Meredith Kercher was brutally murdered (Image: PA)

The newly engaged 27-year-old American had refused to return to Italy from her home in Seattle, Washington, for the trial, saying she would get an unfair trial after her original conviction was overturned in October 2011 and she was released.

But she was found guilty in her absence of Kercher's murder and sentenced to 28 and a half years while Sollecito was handed 25 years.

Job centre worker Ballerini labelled the motive, that Knox supposedly killed Meredith after she complained about the American leaving the bathroom dirty, as 'nonsense'.

“You do not massacre a girl because she complained about a bit of a stink in the bathroom,” she said.

The 48-year-old then hinted she felt the two former lovers did not receive a fair trial saying there was “an atmosphere of guilt” in the deliberation room before it even began.

25 years: Raffaele Sollecito was convicted with Knox, his former lover (Image: AFP)

Meredith, from Couldson, Surrey, had only been in Italy for two months when prosecutors say she was killed in a brutal sex game which went wrong.

Her throat had been slit and she had been sexually assaulted while living in the northern Italian city of Perugia in November 2007.

Knox falsely accused barman Diya "Patrick" Lumumba of the murder and was found guilty of libel and ordered to pay him £19,000 in damages.

Drug dealer Rudy Guede, from the Ivory Coast, was convicted of the killing in a separate trial and sentenced to 30 years, although it was cut to 16 years on appeal.